With the expansion of remotely-accessible public and private networks, users of personal computers are increasingly employing modem equipment to communicate data across public and private telephone lines. The ability to access a wide variety of data resources from nearly anywhere in the world is increasing the value of personal computers as tools for business and increasing the productivity of personal computer users.
As is well known, a "modem" is a device that interfaces a computer to a telephone line, and permits digital data to be communicated across a telephone line by modulating the outgoing digital data so that it is compatible with telephone networks designed to handle analog signals (e.g. speech traffic). Similarly, a modem demodulates incoming analog signals from a telephone line to recover digital data from the analog signals.
The need for telephone systems and equipment to be compatible and interchangeable has led to the development of physical and electrical standards for interfacing telephone equipment, such as modems, to public and private telephone networks. In the United States, the standard physical connector for telephone equipment is the RJ-11, 6-pin miniature module and plug. The standard cable used to connect telephone equipment to a telephone subscriber loop consists of two wires, which are referred to as "tip" and "ring". These two wires are typically connected to the center two leads of an RJ-11 plug at one end of the cable. The RJ-11 plug is typically plugged into an RJ-11 module in a telephone wall jack, which is connected to a telephone subscriber loop. The telephone equipment may be directly connected to the cable, or may be connected to the cable through an RJ-11 plug, or a proprietary connector.
The electrical interface between telephone equipment and the public telephone network has also been standardized, and in the United States, telephone equipment must meet standards promulgated by the Federal Communications Commission (FCC), and by various telephone companies. Manufacturers of telephone equipment must provide an electrical line interface circuit that moderates all signals that are sent over the public telephone network. The line interface circuit protects the public telephone lines and the public telephone network from damage, and thereby helps insure the integrity of transmissions over public telephone systems.
This line interface circuitry is referred to as Data Access Arrangement (DAA) circuitry that typically provides an impedance match between the telephone equipment and the telephone line, and also isolates and protects the telephone equipment from transient signals and other electrical signals that may be present on a telephone line. The DAA also protects the telephone network from any harmful signals that the telephone equipment may generate. Damage to the public telephone network may result, for example, if a modem accidentally injected direct current (DC) power into the telephone line. The FCC mandated DAA circuit does not permit the telephone equipment to transmit such damaging signals to the telephone network.
The DAA circuit typically includes a circuit, such as a diode bridge full-wave rectifier, that permits the DAA to function regardless of the polarity of the voltage on the tip and ring lines of the public telephone network. The DAA circuit also includes a DC-holding circuit and other circuitry that primarily conducts alternating current (AC) (referred to hereinafter as the DAA AC Circuitry). When the telephone equipment is off-hook, the DC-holding circuit keeps the telephone line active by providing a path for DC current while simultaneously presenting a high impedance to AC signals.
The telephone equipment in many offices, hotels, and other large facilities typically is not connected directly to the public telephone network. Rather, these facilities are increasingly wired so that telephone traffic passes through a "private branch exchange" (PBX). A PBX is an automatic switching system that interconnects the telephone equipment within a facility, and acts as a gateway between an internal telephone network and the public telephone network.
Although the connections between a PBX and the public telephone network must meet all of the applicable standards to prevent damage to the public telephone system, the connections between the PBX and internal telephone equipment are not required to meet these standards. Many PBX systems use wires connected to an RJ-11 plug in a manner different from the public telephone system. Some PBX systems use additional wires connected to the RJ-11 plug to transmit control information between the PBX and specialized telephone equipment designed to be used with that particular PBX.
Some PBX systems use the tip and ring lines to send control information, or to supply power to specialized telephone equipment in a manner that may damage telephone equipment that is not specially designed to work with the PBX. Consequently, the DAA circuitry in standard telephone equipment may be damaged or destroyed if the equipment is connected to a PBX having a non-standard wiring arrangement, or nonstandard electrical characteristics.
Because the telephone jacks used with these PBX systems are typically RJ-11 modules, they are indistinguishable from a standard telephone jack. Thus, a personal computer user who connects a modem to a hotel or office telephone system may have no way of knowing if the PBX in use at the hotel or office will damage the modem. As increasing numbers of people connect portable computers equipped with modems to telephone lines in hotel rooms and other facilities that use PBX systems, the likelihood of such damage is considerable.
To prevent damage to the DAA circuitry, many modem manufacturers use a fuse-type device. When the modem is taken "off-hook" while connected to a PBX that supplies power over the tip and ring lines at too high a voltage or current, the fuse burns out, thereby protecting the DAA circuitry. Unfortunately, once a fuse burns out, it must be physically replaced before the modem again will function. Other fuse-like devices, such as circuit breaking devices may also be employed to protect the DAA line interface circuitry.
Use of fuses or other fuse-like devices to protect the DAA circuitry, however, has a number of drawbacks. First, these devices typically are slow to react. Thus, the DAA circuitry must be designed to handle excessive voltage or current until the fuse can react. If a fuse is used, the fuse must be positioned in such a way that it is easily accessible and replaceable, or the modem may have to be returned to the manufacturer, at considerable cost in time and money, to replace the fuse.
Although it is relatively easy to provide access to a fuse on a large external modem, it is much more difficult to provide such access on a compact modem, such as a modem conforming to the PCMCIA specifications, or a mobile modem built into a portable computer. Most fuse-like devices, such as circuit breaking devices are too large to be used in a PCMCIA modem or mobile modem. Because PCMCIA modems and mobile modems typically are used with portable computers, which are most likely to be plugged into an unknown telephone system, some other protection means is needed.
U.S. Pat. No. 5,532,898 to Price describes one attempt to address the foregoing problems. That patent describes current sensing circuitry that generates a signal indicative of excessive line current. The signal is monitored by a microprocessor, that opens a relay when the line current exceeds a predetermined magnitude, thereby protecting the DAA line interface circuitry. The current sensing circuitry comprises numerous additional components separate from other modem and DAA line interface circuitry, and also requires a special purpose "excessive line current" signal. U.S. Pat. No. 5,696,660, also to Price, has an identical written description and claims monitoring a supply voltage present across the telephone line. The patent does not describe circuitry for performing this function, however, but only describes circuitry for monitoring line current.
Line current may not always be a good indicator of potentially damaging power levels on the phone. Some DAA circuitry limits the current at a fixed level, but does not limit the voltage. Because power is the product of current and voltage, even if the current is limited, increases in the voltage may cause the power to rise to levels that may damage the line interface circuitry. Protection circuitry that senses only excessive line current will not detect this condition, and may allow the DAA line interface circuitry to be damaged.
In view of the above, it would be desirable to provide apparatus and methods for protecting DAA line interface circuitry from electrical conditions on a phone line that are appropriate for use in a PCMCIA modem or mobile modem.
It would also be desirable to provide apparatus and methods of protecting DAA line interface circuitry that may be readily integrated with other existing ring detection circuitry, and that requires a minimum of additional components to implement.